About the Slab Thickness Calculator
What is the Slab Thickness Calculator?
Slab depth is usually governed by deflection. This slab thickness calculator applies the IS 456 span-to-effective-depth ratios for one-way, two-way and cantilever slabs, then adds cover and half the bar diameter.
The result is rounded up to the next 5 mm, with a minimum of 100 mm.
How to use the Slab Thickness Calculator
- Shorter Effective Span (m) (sample: 3.5)
- Slab Type (sample: Two-way, continuous (L/d 32, Fe 415))
- Modification Factor for Tension Steel (one-way and cantilever) (sample: 1.4)
- Clear Cover (mm) (sample: 20)
- Main Bar Diameter (mm) (sample: 10)
- Press Calculate Slab Thickness (results also refresh as you type). The result card shows the Recommended Slab Thickness, along with Minimum effective depth, Span / depth ratio used, Self-weight of the slab and Scope.
Formula used
Effective depth d = Span ÷ (Basic L/d ratio × Modification factor)
- Basic ratios = cantilever 7, simply supported 20, continuous 26
- Two-way slabs (≤ 3.5 m, Fe 415) = simply supported 28, continuous 32
- Overall depth = d + clear cover + bar diameter ÷ 2
For preliminary sizing only. Structural designs must be prepared and checked by a qualified structural engineer to IS 456, IS 800 and IS 875.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Shorter Effective Span (m) | 3.5 |
| Slab Type | Two-way, continuous (L/d 32, Fe 415) |
| Modification Factor for Tension Steel (one-way and cantilever) | 1.4 |
| Clear Cover (mm) | 20 |
| Main Bar Diameter (mm) | 10 |
| Result | Value |
|---|---|
| Recommended Slab Thickness | 135 mm |
| Minimum effective depth | 109.4 mm |
| Span / depth ratio used | 32 |
| Self-weight of the slab | 3.38 kN/m² |
| Scope | Deflection control per IS 456 cl. 23.2 / 24.1 |
Tips for using the Slab Thickness Calculator
- Residential roof slabs are commonly 125–150 mm thick.
- Thicker slabs add self-weight – about 0.6 kN/m² for every extra 25 mm.